Spray drying



Patented Feb. 23, 1954 UNITED STATE ATENT OFFICE SPRAY DRYING Application August 3, 1949, Serial No. 108,259

1 Claim. (Cl. 159-48) 1 This invention relates to a method and apparatus for spray drying and more particularly to a novel spray drying method wherein a greater drying capacity is obtained. The principal object of the invention is the provision of a combined furnace and spray drying-chamber installation wherein a greater amount of fuel can be burned in the furnace, and a correspondingly greater amount of heat can be introduced and utilized in the spray drying chamber, without increasing materially the volume of the exhaust gas that must be handled by the cyclone separators and the exhaust fan. A further object is the provision of a spray drying installation wherein a substantially higher average inlet gas temperature can be employed without damage to the spray machine. Still further objects will become apparent from the following detailed description of a preferred embodiment of the invention.

A conventional spray drier that isnow in wide commercial use consists generally of a vertical cylindrical drying chamber having a motor driven spray machine mounted at the top thereof. Hot gases in the form of products of combustion are generated by a gas-burning furnace and are admitted to the spray drying chamber through a flue which surrounds the spray machine. Usually a set of gas-directing vanes is placed in this flue so that the entering gases follow a vertical spiral path,

the solution or slurry to be spray dried being disill charged laterally into this spiral gas stream.

In installations of this type the temperature of the hot gases must ordinarily be restricted to about 750 or thereabouts, since the use of hotter gases increases greatly the danger of damaging the flues and the spray machine. This relatively low inlet gas temperature limits the quantity of liquid that can be evaporated in the spray drying chamber, and consequently limits the amount of product that can be produced in equipment of any given size. Ordinarily the mixture of gases and spray dried material issuing from the spray drying chamber is passed into a set of cyclone separators, Where the solid material is separated from the gases, after which the gases pass through a blower or fan and are discharged to the atmosphere. In order to increase the capacity of the spray drying equipment a correspondingly increased volume of gases must be handled in the drying chamber, the cyclone sepa- L rators and in the blower; in other words, the gas volume of the entire installation must be increased.

By the present invention, an increase in the drying capacity of the equipment is obtained H without increasing the volume of the exhaust gas through the cyclones and fan. This is done by introducing a portion of the hot gases at a higher temperature into the lower part of the spray drying chamber where they are out of contact with the spray machine. Gases having any desired temperature above about 750 F., or above the temperature limitation imposed by danger of damaging the spray machine, may be used.

In accordance with another feature of the invention, two gas streams of different temperatures are withdrawn from the same furnace by the use of secondary air or other diluting gases. In this arrangement a second flue or duct extends between a side outlet in the furnace and the lower part of the spray drying chamber. An inlet for secondary air is provided in the furnace above or beyond this hot gas outlet, so that the remaining gases in the furnace can be cooled to the desired temperature on the order of 750 F. at which they can be used for introduction at the top of the spray drying chamber.

The invention will be further described with v reference to the accompanying drawing in which a cylindrical side wall 2, a conical bottom portion.

3 and a generally flat top 4 in which a central well structure 5 is mounted. A spray machine 6, consisting of an electric motor 1 driving a vaned spray wheel 8 of conventional design is mounted in the cylinder 5. An outer vertical duct [0 con.- taining gas directing vanes ll may surround the central well structure 5. A generally cylindrical gas flue [2, open at its bottom, also surrounds the central well 5 and provides communication between the interior of the spray drying chamber and a gas duct 13 leading to the top of the furnace M. The conical bottom 3 of the spray drying chamber terminates in a gas outlet pipe l5 which leads to a set of cyclone separators and then to an exhaust blower or fan, these being of conventional design.

The furnace It consists generally of a preferably bricl; lined cylinder it which may be divided into a lower primary combustion zone i1 and an upper dilution zone 18. Gas burners 19 are provided near the bottom of the combustion zone together with an inlet 20 for the introduction of primary air. A hot gas outlet 2!, which is preferably lined with firebrick or other heat-resistant material, connects the primary combustion zone H with a lower portion 23 of the spray drying chamber. It will be understood that the location of the pipe 2| with respect to the spray drying chamber is not necessarily at the exact level shown on the drawing, but may be at any level below that of the spray wheel 8.

Above the hot gas outlet pipe 2| the furnace I4 is provided with an inlet 25 for secondary air. Inasmuch; as. the secondary air isgusedzforz the purposemf; diluting and thereby coolingthe gases in this portion of the furnace, it will be understood that a non-combustible gas other than airy;

may also be used for this purpose if desired; thus,

for example, a portion of the exhaust gases from the main blower may be recirculated to-theinlet 25. Above the level of the secondary-1ain1inlet25f a gas outlet pipe 26 is located, this pipe connecte ing the dilution zone I8 withtheiduct. ltthat leads to the top of the spray drying chamber. A suitable stack 2?, controlled by a dampen. ZLmay. be provided at the top ofthe furnace if desired. Allrofntherairhinletand,gas outletpipesleadinggta and .from the:iurnace.areiproyidedlwith;dampersflerably about. 15.0.091 and.introducedtinto.the. lowermportion 23..Loi1thespray, drying. chamber...

Theremainder. ofhthe hot. gases; within. the .fur: naceJ 4 are. diluted and...'coolfed'..by, secondary... air-.-

or other gas introduced throu h; the, inlet. 25..

whereby, the temperaturev is. reduced to...750..' F101: lower; Gases, having this,temperature.,are,..withY-. drawn from the dilution, zone. flilithrougliv the. outlet 26 and pass. throughlpipe. I3Tandjdistributingilue ,l 2' into the topof the spray drying chain.-v her, ,where they I form a rotating, gas blanket;intov whichthe, material to ,be dried is sprayed-by the spray wheel 8.; The. resulting mixture passes. downwardly through the chamber while. addij tiona l hot gas, is introduced 'at1a lower level through the. pipe 2'!" at a temperature well above. 750" and preferably at 1000-1"5'00'F,

As a result-of the introduction of drying; gases atj'two different levels inithesprayclryi ng cham= her by, the procedure described, a; number of' op: erating advantages may be obtained; Thepartia 4 cles of liquid material thrown off by the spray wheel 8 first encounter a smaller flow of gas, which may also be at lower temperature, than in the case of prior operating conditions. Under these circumstances they will travel a greater distance from the atomizing wheel before they become solidified or set by drying, and therefore dried particles-of improved sphericity are obtained. Funthermore, ashas been: explained, the evaporating capacity of the spray drier is greatly increased; by employing a ratio of hot gas at 500 F. (introduced through the pipe 2|) to cool=-gas at 750 F. (introduced through the pipe I30. such..;tha-t.an average inlet temperature of canibegevaporatedrthan when all of the gas is introduced at 750 F. through the pipe 13. This improvement in-evaporation is obtained with the sametoutlet temperature (about 230 F.) in pipe l5 and with the same volume of drying gases in the. cyclone separators and exhaust .fan.

What ricla'im is'z.

A method. of ifspray drying; which: comprises. burning a hydrocarbon. fuel with primary aii' 'tol form a body of primary combustion having; a vtezmc erature above 1000? F1; withdrawingapom tion" of said'primary v combustion gases and diluting the remainder withcooler gas to form 'a body. ofv hot secondary combustion gashavin'g a.tern1-. perature nothigher than about'750? EL, introduce; ing stream of said; hot secondary combustion downwardly into theitopjof'achamberwhfle dispersing. therein" a spraymf. liquid material to. bepdried and'passingthe resultingmix'ture downs wardly through said ichamher and introducing thereinto. the. withdrawn portion. of said primary combustion gases'ata level below that'at which said liquid" material was introduced;

WILLIAM; .L. .SPALDING.

References Cited in the --file' of" thispatient UN ITEDJ. STATES EATENHZSQ- 

